Latitudinal and Seasonal Variation

An actual atmospheric profile can vary somewhat from the 1976 U.S. Standard Atmosphere. The amount of variation will depend on the latitude, day of the year, and even the time of day. Figure 2.4a corresponds to Figure 2.10 in U.S. Standard Atmosphere Supplements, 1966, which utilizes the 1962 U.S. Standard Atmosphere as the reference. The difference between the 1962 and 1976 models are small within the mesosphere, so the comparison also applies to the 1976 U.S. Standard Atmosphere.

Figure 2.4: a) Typical percentage variations in air-density relative to the U.S. Standard Atmosphere are shown as function of altitude for different latitudes and times of year. b) The approximate diurnal variation in air-density is shown. The curves containing horizontal stripes correspond to density maxima which occur at night. Significant errors may exist in the diurnal density variation estimates above 40 km altitude. (From U.S. Standard Atmosphere Supplements, 1966).

Figure 2.4a shows how the air-density varies for 3 different latitudes as well as different times of the year. Generally, the higher the latitude, the more seasonal variation there will be.

Sprites within the U.S. are most numerous in July and August (Lyons et al., 1999), which is near the time when the air density within the mesosphere is at a maximum (Figure 2.4a). This would imply that at any given altitude, the breakdown field, $E_k$, also increases during the summer. For instance, during the summer season $E_k$ would be $\sim\,$20% greater than in the spring and fall seasons and $\sim\,$40% greater than in winter at 80 km altitude and 45$^\circ$N latitude. However, the detailed comparisons of experiment with theory in this dissertation will focus primarily on sprites which occurred in the fall (see Section 5.3). The variation from the standard atmosphere for the fall season will be small, particularly in comparison to other sources of error.

The approximate diurnal variation of air density within the mesosphere is shown in Figure 2.4b. The nighttime air density near 80 km altitude can be as much as 25% less than the daytime density. However, nighttime rocket soundings conducted from White Sands, NM, (U.S. Standard Atmosphere Supplements, 1966, p. 31) measured air-densities at 60-80 km altitude to be between $-$3% to $+$8% of the standard atmosphere air densities. Thus, the 1976 U.S. Standard Atmosphere model should be adequate for theoretical calculations (Section 2.5) which will be compared with experimental observations from Langmuir Laboratory, NM.